Many of automobile parts such as chassis and body structures are conventionally produced by press-forming steel sheets having prescribed strength at a room temperature. In recent years, the weight reduction of automobile body has been desired from the viewpoint of the preservation of global environment. Thus, there have been continuous efforts to decrease the thickness of steel sheet used for automobile parts by increasing the strength of the steel sheet. However, increasing the strength of steel sheet is accompanied by a decrease in press-formability of the steel sheet and it is frequently difficult to press-form such a steel sheet into automobile parts with desired shapes.
To remedy this difficulty, Patent Literature 1 proposes a technique in which a steel sheet that has been heated is press-formed with a mold composed of a die and a punch while being simultaneously rapidly cooled. This technique, called a hot press-forming, realizes good press-formability of a high strength steel sheet. This hot press-forming, however, involves heating of a steel sheet to a high temperature of about 950° C. before the press-forming. As a result, scales (iron oxide) are formed on the surface of the steel sheet. Such scales are peeled off during the hot press-forming to damage the mold or damage the surface of hot press-formed parts. Further, scales remaining on the surface of parts deteriorate the appearance or decrease the paint adhesiveness. Thus, the surface of such parts is usually descaled by treatments such as pickling or shot blasting. However, the implementation of these treatments makes the manufacturing steps complicated and decreases the productivity. Such automobile parts as chassis and body structures require excellent corrosion resistance, but the hot press-formed parts manufactured through the above treatments show significantly insufficient corrosion resistance due to the fact that they are not provided with an anticorrosion film such as a coating layer.
Thus, there has been a demand for a hot press-forming technique which can suppress the formation of scales during heating before hot press-forming and also can increase the corrosion resistance of hot press-formed parts. This has led to proposals of steel sheets having a film such as a coating layer on the surface, and hot press-forming methods using such steel sheets. For example, Patent Literature 2 discloses a method for manufacturing a hot press-formed part with excellent corrosion resistance in which a steel sheet coated with Zn or Zn-based alloy is hot press-formed so that Zn—Fe-based compound or Zn—Fe—Al-based compound is formed on the surface of the part. Patent Literature 3 discloses a hot press-formed article (part) with excellent formability, weldability and corrosion resistance that is obtained by heating a galvannealed steel sheet at 700 to 1000° C. for 2 to 20 minutes and hot press-forming the steel sheet to form a coating layer containing Fe—Zn solid solution phase on the surface of the article.